Measurements are made in different physiological states of the concentrations of key enzymes controlling the rates of metabolic degradation of tyrosine, phenylalanine and tryptophan, and of other enzymes whose concentrations in tissues change. Important categories of the changes to be studied are those induced by substrates of the enzymes and by hormones. Evidence will be sought that the changes are reflected in altered physiological functions. The chemical mechanisms responsible for the observed changes will be investigated, including those responsible for activation of the gene, synthesis of the specific protein, and degradation of the specific protein.